Apparatus and method for producing core rod of optical fiber

ABSTRACT

An apparatus and a method for producing a depressed-cladding core rod of an ultra-low water peak optical fiber, the apparatus including a core rod component, an inner cladding casing component, a high temperature heat source, chucks, a rotary joint, an external gas pipe, and a pressure controlling pipe. The core rod component is produced by fusing and splicing a core layer rod and a first hollow shaft together. The inner cladding casing covers the outside of the core layer rod. The inner cladding casing component is produced by fusing and splicing an inner cladding casing and a second hollow shaft together. These two hollow shafts are clamped in two chucks of a glass lathe. A high temperature heat source is arranged outside the inner cladding casing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International PatentApplication No. PCT/CN2013/072603 with an international filing date ofMar. 14, 2013, designating the United States, now pending, and furtherclaims priority benefits to Chinese Patent Application No.201210546942.0 filed Dec. 17, 2012. The contents of all of theaforementioned applications, including any intervening amendmentsthereto, are incorporated herein by reference. Inquiries from the publicto applicants or assignees concerning this document or the relatedapplications should be directed to: Matthias Scholl P. C., Attn.: Dr.Matthias Scholl Esq., 245 First Street, 18th Floor, Cambridge, Mass.02142.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a method for producinga depressed-cladding core rod of an ultra-low water peak optical fiber.The method avoids contamination of the interface and the inside of thecore rod by moisture and impurities in the air.

2. Description of the Related Art

Process of producing optical fiber normally includes three steps. Thefirst step is producing the core rod. The core rod usually consists oftwo parts: the core layer and the inner cladding layer. The methods ofproducing a core rod mainly include vapor axial deposition method (VAD),modified chemical vapor deposition method (MCVD), plasma chemical vapordeposition method (PCVD), and outside vapor deposition (OVD). The secondstep is adding an outer cladding layer outside the core rod. The thirdstep is drawing to produce the optical fiber.

The methods for adding an outer cladding layer outside the core rodmainly include rod-in-tube method (RIT), rod-in-cylinder method (RIC),and outer cladding deposition method. The rod-in-tube method isinserting the core rod into a casing tube, fusing the casing tube andthe core rod together in a high temperature to form a solid preform. Therod-in-cylinder method is also inserting the core rod into a casingtube, while the fusing process of the casing tube accompanies with aprocess of drawing. The outer cladding deposition method takes advantageof technologies such as soot cladding, advanced plasma vapor depositionmethod (APVD), and plasma outside vapor deposition method (POD) to addan outer cladding outside the core rod to produce a solid preform. Inorder to control the production cost of the optical fiber, the fractionof the impurities (especially the moisture) in the material of the outercladding layer is usually higher than that in the core rod, and thephysical and chemical properties of the outer cladding are not so goodas those of the core rod. Furthermore, deposition of the outer claddinglayer is usually performed in a high temperature that affects thequality of the core rod. In order to eliminate the influence of thematerial and the deposition process on the quality of the core rod, thethickness of the inner cladding layer is usually not smaller than 10times the wavelength of the propagating light. To satisfy this demand,the ratio of the outer diameter of the inner cladding layer to the outerdiameter of outer cladding layer is usually not smaller than 4.

Among the methods of producing a core rod, VAD method is widely usedbecause it has fewer requirements on the purity of raw materials, arelative high deposition speed, and better dehydration effects. VADmethod is capable of continuously producing large-sized preform, whichhas a refractive index profile that does not have depression in thecenter. However, during the processes of dehydroxylation and sintering,the fluorine doped in the inner cladding layer easily permeates the corelayer or evaporates in a high temperature to be taken away by the drygas (Cl₂ and He). Thus, in the core rod produced by traditional VADmethod, the difference of the depressed refractive indexes usually doesnot reach −0.003 (approximate −0.2% of n(SiO₂), in which n(SiO₂)represents the refractive index of SiO₂). Therefore, it is difficult toachieve a complex refractive index profile (e.g., that of an innercladding layer in shape of concave and convex in turn) using traditionalVAD method. This disadvantage limits the application of traditional VADmethod to only production of single model optical fiber having a simplerefractive index profile without deep depression, such as G652,G657A1/G657A2, etc.

SUMMARY OF THE INVENTION

In view of the above-described problems, it is one objective of theinvention to provide an apparatus and a method for producing adepressed-cladding core rod of an ultra-low water peak optical fiber.The apparatus and the method use the core rod produced by VAD or OVDmethod and the casing produced by MCVD, PCVD, or POD method to producedepressed-cladding core rod for an ultra-low water peak optical fiber.

To achieve the above objective, the following technical schemes areprovided.

The apparatus for producing a depressed-cladding core rod of anultra-low water peak optical fiber comprises a core rod component, aninner cladding casing component, a high temperature heat source, chucks,a rotary joint, an external gas pipe, and a pressure controlling pipe.

The core rod component is produced by fusing and splicing a core layerrod and a hollow shaft together. A plurality of vents are drilled on thehollow shaft at the end near the core layer rod. The inner claddingcasing component is produced by fusing and splicing an inner claddingcasing and a hollow shaft together. The inner diameter of the innercladding casing is at least 0.3 mm larger than the outer diameter of thecore layer rod. The inner cladding casing covers the outside of the corelayer rod and the distance between the inner cladding casing and thecore layer rod is 0.15 to 5 mm.

The hollow shaft of the core rod component and the hollow shaft of theinner cladding casing component are clamped respectively in two chucksof a glass lathe. The outer end of the hollow shaft of the core rodcomponent is connected to an external gas pipe via the rotary joint.Various gases are transported through the external gas pipe. These gasesinclude, while not limited to, purge gas, dry gas, and etching gas. Thepurge gas may be purified N₂, O₂, He, etc. The dry gas may be Cl₂. Theetching gas may be CF₄, C₂F₆, SF₆, etc. The external gas pipe isconnected to the pressure controlling pipe and a scrubber. The scrubberis adapted to treat exhaust gas. The outer end of the hollow shaft ofthe inner cladding casing component is connected to the scrubber and isadapted to control the pressure within the gap between the innercladding casing and the core layer rod. The outer end of the innercladding casing is connected to the hollow shaft of core rod componentin an airtight manner. The position of connection needs to be outsidethe vents to ensure that the external gas pipe is communicated with thegap between the inner cladding casing and the core layer rod. Theairtight manner may be mechanical airtight or fusion-sealing manner inhigh temperature. Fusion-sealing manner is used.

The high temperature heat source is arranged on the outside of the innercladding casing. The high temperature heat source may be a gas blowtorchor furnace.

The refractive index profile of the preform produced by theabove-mentioned apparatus includes a core layer, an inner claddinglayer, and an outer cladding layer.

The above-mentioned core layer is produced by VAD or OVD method. Itincludes a core layer and a small fraction of inner cladding. In thecore layer rod, the cladding-to-core diameter ratio may be less than 4,and may be close to 1. The inner cladding layer which has a particularrefractive index profile is an inner cladding casing produced by othermethods. The outer cladding layer is produced by casing tube ordeposition method.

A method for producing a depressed-cladding core rod of an ultra-lowwater peak optical fiber, including the following steps:

-   -   1) producing a core rod component: using a glass lathe to fuse        and splice a core layer rod and a core rod hollow shaft        together;    -   2) producing an inner cladding casing component: using the glass        lathe to fuse and splice an inner cladding casing and a casing        hollow shaft together, the inner diameter of the inner cladding        casing is at least 0.3 mm larger than the outer diameter of the        core layer rod;    -   3) disposing the core rod hollow shaft and the casing hollow        shaft respectively in two chucks of the glass lathe, the        distance between the inner cladding casing and the core layer        rod is 0.15 to 5 mm;    -   4) cutting off the connections among a pressure controlling        pipe, a scrubber, and a vacuum pump, then connecting the core        rod hollow shaft to an external gas pipe via a rotary joint, and        then connecting the casing hollow shaft to the scrubber;    -   5) connecting the inner cladding casing to the core rod hollow        shaft hermetically;    -   6) turning on the glass lathe, rotational speeds of the two        chucks are 20 to 100 rpm;    -   7) transporting a first mixture gas comprising a purge gas and a        dry gas to the core rod hollow shaft at room temperature for        approximate 2 minutes via the external gas pipe, a flow rate        ratio of the purge gas to the dry gas is from 20:1 to 80:1;    -   8) while transporting the first mixture gases to the core rod        hollow shaft continuously in a flow rate ratio of the purge gas        to the dry gas of from 20:1 to 80:1, moving a high temperature        heat source back and forth twice to warm the inner cladding        casing, a speed of moving the high temperature heat source is 50        to 200 mm per minute and a temperature of an inner wall of the        inner cladding casing is approximately 300° C. to 800° C.;    -   9) transporting a second mixture gas comprising the purge gas        and an etching gas to the core rod hollow shaft to clean the        interfaces of the glass, the flow rate ratio of the purge gas to        the etching gas is 5:1 to 20:1; then moving the high temperature        heat source from where the second mixture gas flows in to where        the second mixture gas flows out so that the etching gas        decomposes and reacts with the glass at a temperature of        1200° C. to 1900° C., and the moving speed of the high        temperature heat source is 20 to 100 mm per minute; and then        adjusting a pressure within the gap between the inner cladding        casing and the core layer rod to prevent the outer diameter of        the inner cladding casing from shrinking or expanding at a high        temperature, the pressure within the gap between the inner        cladding casing and the core layer rod is from 30 Pa to 400 Pa;    -   10) transporting the first mixture gas, the flow rate ratio of        the purge gas to the dry gas is 20:1 to 80:1, and the pressure        within the gap between the inner cladding casing and the core        layer rod is lower than 60 Pa; then heating the inner cladding        casing with the high temperature heat source at the end where        the gases flow out, i.e., at the end near the casing hollow        shaft, to induce shrinkage of the inner cladding casing; and        then opening the pressure controlling pipe and fusing the inner        cladding casing and the core layer rod together at the end where        the gases flow out when the gap between the inner cladding        casing and the core layer rod disappears;    -   11) transporting the first mixture gas, the flow rate ratio of        the purge gas to the dry gas is 20:1 to 80:1, the tube pressure        is controlled according to the thickness of the inner cladding        casing in a range of from +60 Pa to −99 kPa; then opening the        connection between the pressure controlling pipe and the vacuum        pump when a negative pressure is required, and then moving the        high temperature heat source towards the end where the gases        flow in to heat the inner cladding casing and to fuse the inner        cladding casing with the core layer rod, the speed of moving the        high temperature heat source is 5 to 100 mm per minute; and    -   12) controlling a vacuum degree of the gases in the external gas        pipe and the pressure controlling pipe in a range of from +60 Pa        to −99 kPa, controlling the temperature of high temperature heat        source in a range of 1200° C. to 1900° C., and controlling the        speed of moving the high temperature heat source to be 5 to 100        mm per minute to fuse the inner cladding casing with the core        layer rod to provide a depressed-cladding core rod not having        interfacial air bubbles and air lines and having an acceptable        roundness.

The key step is combining the inner cladding casing and the core layerrod to form a solid core rod by a modified rod-in-tube method. Intraditional rod-in-tube method, because the inner surface of the innercladding and the outer surface of the core layer rod are directlyexposed to the air, moisture and other impurities in the air willpermeate and pollute the inner cladding casing and the core layer rodwhen they are heated under a high temperature, which affects the qualityof the optical fiber and causes an increased attenuation of the opticalfiber at 1383 nm. The less the cladding-to-core diameter ratio is, themore largely the disadvantages of the traditional rod-in-tube methodaffect the quality of the optical fiber. The conventional rod-in-tubemethod is modified in order to get rid of the moisture and otherimpurities in the gap.

Advantages of the invention are summarized below: The core layer and theinner cladding layer are produced by different apparatuses and methods.The core rod produced by combining such core layer and inner claddinglayer together has deeply depressed or complex refractive index profile.The attenuation of the optical fiber which is formed by drawing theproduced core rod is less than 0.33 dB/km at 1383 nm, which means theoptical fiber is suitable for long distance transmission. This largelyreduces the cost on optical amplifiers and repeaters which are requiredin long distance transmission. Furthermore, it apparently reduces thecost of maintenance. The preform produced by the method and apparatus ofthe invention can be processed into optical fibers having an ultra-lowrefractive index and ultra-low water peaks and suitable for longdistance transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the refractive index profile of thedepressed cladding ultra-low water peak optical fiber;

FIG. 2 is a schematic diagram of the core rod component of theinvention;

FIG. 3 is a schematic diagram of the inner cladding casing component ofthe invention;

FIG. 4 is a schematic diagram of the process of assembling tubes,getting rid of moisture, and scrubbing interface; and

FIG. 5 is a schematic diagram of the modified rod-in-tube method.

In the drawings, the following reference numbers are used: 1. corelayer; 2. inner cladding layer; 3. outer cladding layer; 4. hollow shaftof the core rod component; 5. core layer rod; 6. inner cladding casing;7. hollow shaft of the inner cladding casing component; 8. hightemperature heat source; 9. chuck; 10. rotary joint; 11. external gaspipe; and 12. pressure controlling pipe.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To further illustrate the invention, an apparatus and a method ofproducing a depressed-cladding core rod of an ultra-low water peakoptical fiber are described below. It should be noted that the followingexamples are intended to describe and not to limit the invention.

Detailed description of embodiments of the invention will be given belowin conjunction with accompanying FIGS. 1-5.

As shown in FIGS. 2-5, the apparatus of producing a depressed-claddingcore rod of an ultra-low water peak optical fiber includes a core rodcomponent, an inner cladding casing component, a high temperature heatsource 8, chucks 9, a rotary joint 10, an external gas pipe 11, and apressure controlling pipe 12.

As shown in FIG. 2, the core rod component is produced by fusing andsplicing the core layer rod 5 and the hollow shaft 4 together. Aplurality of vents is drilled on the hollow shaft 4 at the end near thecore layer rod 5. As shown in FIG. 3, the inner cladding casingcomponent is produced by fusing and splicing an inner cladding casing 6and a hollow shaft 7 together. The inner diameter of the inner claddingcasing 6 is at least 0.3 mm larger than the outer diameter of the corelayer rod 5. The inner cladding casing 6 of the inner cladding casingcomponent covers the outside of the core layer rod 5, and the distancebetween the inner cladding casing 6 and the core layer rod 5 is 0.15 to5 mm.

The hollow shaft 4 of the core rod component and the hollow shaft 7 ofthe inner cladding casing component are clamped respectively in twochucks 9 of a glass lathe. The outer end of the hollow shaft 4 of thecore rod component is connected to an external gas pipe 11 via a rotaryjoint 10. Various gases are transported through the external gas pipe11. These gases include, while not limited to, purge gas, dry gas, andetching gas. The purge gas may be purified N₂, O₂, He, etc. The dry gasmay be Cl₂. The etching gas may be CF₄, C₂F₆, SF₆, etc. The external gaspipe 11 is connected to the pressure controlling pipe 12 and a scrubber.The scrubber is adapted to treat exhaust gases. The outer end of thehollow shaft 7 of the inner cladding casing component is connected tothe scrubber and is adapted to control the pressure between the innercladding casing 6 and the core layer rod 5. The outer end of the innercladding casing 6 in the inner cladding casing component is connected tothe hollow shaft 4 of core rod component in an airtight manner, and theconnection is placed outside the vents to ensure that the external gaspipe 11 is communicated with the gap between the inner cladding casing 6and the core layer rod 5. The airtight manner may be mechanical airtightor fusion-sealing manner in high temperature. In FIG. 4, fusion-sealingmanner is used.

The high temperature heat source 8 is arranged on the outside of theinner cladding casing 6. The high temperature heat source may be a gasblowtorch or furnace.

The refractive index profile of the preform produced by above-mentionedapparatus includes the core layer 1, the inner cladding layer 2, and theouter cladding layer 3.

The above-mentioned core layer 1 is the core layer rod 5 produced by VADor OVD method. It includes the core layer 1 and a small fraction ofinner cladding. In the core layer rod, cladding-to-core diameter ratiomay be less than 4, and may be close to 1. The inner cladding layer 2which has a particular refractive index profile is an inner claddingcasing 6 produced by other methods. The outer cladding layer 3 isproduced by deposition method or is produced from casing.

The key step is combining the inner cladding casing 6 and the core layerrod 5 to form a solid core rod by a modified rod-in-tube method. Intraditional rod-in-tube method, because the inner surface of the innercladding 6 and the outer surface of the core layer rod 5 are directlyexposed to the air, moisture and other impurities in the air willpermeate and pollute the inner cladding casing 6 and the core layer rod5 when they are heated under a high temperature, which affects thequality of the optical fiber and causes an increased attenuation ofoptical fiber at 1383 nm. The less the cladding-to-core diameter ratiois, the more largely the disadvantages of the traditional rod-in-tubemethod affect the quality of the optical fiber. The conventionalrod-in-tube method is modified in order to get rid of the moisture andother impurities in the gap.

In the modified method, the two components are first produced. FIG. 2shows the core rod component produced by fusing and splicing a corelayer rod 5 and a hollow shaft 4 together. A plurality of vents isdrilled on the hollow shaft 4 at the end near the core layer rod 5. Asshown in FIG. 3, the inner cladding casing component is produced byfusing and splicing an inner cladding casing 6 and a hollow shaft 7together. The inner diameter of the inner cladding casing 6 is at least0.3 mm larger than the outer diameter of the core layer rod 5.

The method may be performed in a horizontal or vertical glass lathe.FIG. 4 is a schematic diagram showing the process of assembling the twocomponents and purging the gas between the two components. Afterassembly, the gap between the inner cladding casing 6 and the core layerrod 5 is 0.15-5 mm. The high temperature heat source 8 in the figure maybe, while not limited to, a gas blowtorch or furnace. FIG. 5 is aschematic diagram of the modified rod-in-tube method.

As shown in FIG. 4, during the process of assembling, the hollow shaft 4of the core rod component and the hollow shaft 7 of the inner claddingcasing component are clamped respectively in two chucks 9 of the glasslathe. The outer end of the hollow shaft 4 of the core rod component isconnected to the external gas pipe 11 via a rotary joint 10. Variousgases are transported through the external gas pipe 11. These gasesinclude, while not limited to, purge gas, dry gas, and etching gas. Thepurge gas may be purified N₂, O₂, He, etc. The dry gas may be Cl₂. Theetching gas may be CF₄, C₂F₆, SF₆, etc. The external gas pipe 11 isconnected to the pressure controlling pipe 12 and a scrubber. Thescrubber is adapted to treat exhaust gas. The outer end of the hollowshaft 7 of the inner cladding casing component is connected to thescrubber and is used to control the pressure within the gap between theinner cladding casing 6 and the core layer rod 5. The outer end of theinner cladding casing 6 in the inner cladding casing component needs tobe connected to the hollow shaft 4 of core rod component in an airtightmanner. The connection position needs to be placed outside the vents toensure that the external gas pipe 11 is communicated with the gapbetween the inner cladding casing 6 and the core layer rod 5. Theairtight manner may be mechanical airtight or fusion-sealing manner inhigh temperature. Fusion-sealing manner is used.

Further instruction of the invention is provided below with an exemplaryembodiment.

A core layer rod is produced by VAD method. After the processes ofsintering, extending, and surface treatment, the outer diameter of thecore layer rod 5 is 16.2 mm, and the length thereof is 1450 mm. Therefractive index profile of the core layer rod 5 is measured, and thecladding-to-core diameter ratio of the core layer rod 5 is approximately1.78. The inner cladding casing 6 is produced from a material having alow hydroxyl content (OH: approximately 0.2 ppm). The sectional area ofthe inner cladding casing 6 is 890 mm² and the inner diameter thereof is17.6 mm.

The inner cladding casing 6 and the core layer rod 5 are fused togetherto produce a core rod by modified rod-in-tube method, including thefollowing steps:

Referring to FIG. 2 and FIG. 3, the following steps 1 and 2 are theprocesses of producing the core rod component and the inner claddingcasing component. Referring to FIG. 4, the following steps 3 to 9 arethe processes of assembling and purging. Referring to FIG. 5, thefollowing steps 10 to 12 are the processes of fusing. The hightemperature heat source 8 in the glass lathe is a blowtorch withoxyhydrogen flame. The flow rate ratio of oxygen to hydrogen is 1:2.

A method for producing a depressed-cladding core rod of an ultra-lowwater peak optical fiber, including the following steps:

1) Producing the core rod component: using the glass lathe to fuse andsplice the core layer rod 5 and the hollow shaft 4 together.

2) Producing the inner cladding casing component: using the glass latheto fuse and splice the inner cladding casing 6 and the hollow shaft 7together, the inner diameter of the inner cladding casing 6 is at least0.3 mm larger than the outer diameter of the core layer rod 5.

3) Clamping the core rod component and the inner cladding casingrespectively in two chucks 9 of the glass lathe and disposing the innercladding casing 6 outside the core layer rod 5, the distance between theinner cladding casing 6 and the core layer rod 5 is 0.15 to 5 mm.

4) Cutting off the connection among the pressure controlling pipe 12,the scrubber, and a vacuum pump, then connecting the hollow shaft 4 ofthe core rod component to the external gas pipe 11 via the rotary joint10, and then connecting the hollow shaft 7 of the inner cladding casingto the scrubber.

5) Connecting the inner cladding casing 6 to the hollow shaft 4 of thecore rod component hermetically.

6) Turning on the glass lathe, rotational speeds of the two chucks are20 to 100 rpm.

7) Transporting a first mixture gas comprising a purge gas and a dry gasto the core rod hollow shaft at room temperature for approximate 2minutes via the external gas pipe 11, a flow rate ratio of the purge gasto the dry gas is from 20:1 to 80:1.

8) While transporting the first mixture gas to the core rod hollow shaftcontinuously in a flow rate ratio of the purge gas to the dry gas offrom 20:1 to 80:1, moving the high temperature heat source 8 back andforth twice to warm the inner cladding casing 6 properly, a speed ofmoving the high temperature heat source 8 is 50 to 200 mm per minute anda temperature of an inner wall of the inner cladding casing 6 isapproximately 300° C. to 800° C.

9) Transporting a second mixture gas comprising the purge gas and anetching gas to the core rod hollow shaft to clean the interfaces of theglass, the flow rate ratio of the purge gas to the etching gas is 5:1 to20:1; then moving the high temperature heat source 8 slowly from wherethe second mixture gases flow in to where the second mixture gases flowout to heat the inner surface of the inner cladding casing 6 to atemperature of 1200° C. to 1900° C. so that the etching gas decomposesand reacts with the glass, and the moving speed of the heat source is 20to 100 mm per minute; and then adjusting the pressure within the gapbetween the inner cladding casing 6 and the core layer rod 5 forpreventing the outer diameter of the inner cladding casing 6 fromshrinking or expanding at high temperature, the pressure within the gapbetween the inner cladding casing 6 and the core layer rod 5 iscontrolled to 30 Pa to 400 Pa.

10) Transporting the first mixture gas to the core rod hollow shaft, theflow rate ratio of the purge gas to the dry gas is 20:1 to 80:1, and thepressure within the gap between the inner cladding casing 6 and the corelayer rod 5 is lower than 60 Pa at this moment; then heating the innercladding casing 6 with the high temperature heat source 8 at the endwhere the gases flow out, i.e., at the end near the hollow shaft 7, toinduce shrinkage of the inner cladding casing; and then opening thepressure controlling pipe 12 and fusing the inner cladding casing andthe core layer rod together when the gap between the inner claddingcasing 6 and the core layer rod 5 almost disappears.

11) Continuously transporting the first mixture gas comprising the purgegas and the dry gas, the flow rate ratio of the purge gas to the dry gasis 20:1 to 80:1, the tube pressure is controlled according to thethickness of the inner cladding casing in a range of from +60 Pa to −99kPa; then opening the connection between pressure controlling pipe 12and the vacuum pump when a negative pressure is required, and thenmoving the high temperature heat source 8 towards where the gases flowin to heat the inner cladding casing and to fuse the inner claddingcasing 6 with the core layer rod 5, the speed of moving the hightemperature heat source is 5 to 100 mm per minute.

12) controlling a vacuum degree of gases in the external gas pipe 11 andthe pressure controlling pipe 12 in a range of from +60 Pa to −99 kPa,controlling the temperature of high temperature heat source 8 in a rangeof 1200° C. to 1900° C., and controlling the speed of moving the hightemperature heat source to be 5 to 100 mm per minute to fuse the innercladding casing 6 with the core layer rod 5 to provide adepressed-cladding core rod having no interfacial air bubbles and airlines and having an acceptable roundness. Therefore, adepressed-cladding core rod of ultra-low water peaks is produced.

The above-mentioned purge gas may be purified N₂, O₂, or He; the dry gasmay be Cl₂; and the etching gas may be CF₄, C₂F₆, or SF₆.

The refractive index profile of the core rod produced according to theinvention is shown in FIG. 1. The core rod having a cladding-to-corediameter ratio of 4.06 is used to produce preform of an optical fiber bydepositing an outer cladding with soot method. The attenuation of theoptical fiber which is produced by drawing such core rod is less than0.33 dB/km at 1383 nm. In international standard ITU-T G. 652.D, theupper limit of the attenuation at 1383 nm is 0.35 dB/km Thus, theoptical fiber produced by this modified rod-in-tube method satisfies thedemand for a low water peak optical fiber. It is suitable forlong-distance transmission and largely reduces the cost on opticalamplifiers and repeaters which are usually required in long distancetransmission. Furthermore, it would apparently reduce the cost ofmaintenance.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and therefore, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

The invention claimed is:
 1. An apparatus for producing adepressed-cladding core rod of an ultra-low water peak optical fiber,the apparatus comprising: a core rod component, an inner cladding casingcomponent, a high temperature heat source, two chucks, a rotary joint,an external gas pipe, and a pressure controlling pipe, wherein: the corerod component is produced by fusing and splicing a core layer rod and acore rod hollow shaft together; a plurality of vents is arranged at anend of the core rod hollow shaft adjacent to the core layer rod; theinner cladding casing component is produced by fusing and splicing aninner cladding casing and a casing hollow shaft together; the innercladding casing is disposed outside the core layer rod of the core rodcomponent; the core rod hollow shaft and the casing hollow shaft aredisposed in the two chucks, respectively; an outer end of the core rodhollow shaft is connected to the external gas supply pipe via the rotaryconnector for transporting gases through the external gas pipe to thecore rod hollow shaft; the pressure controlling pipe is connected to theexternal gas pipe and a scrubber; an outer end of the casing hollowshaft is connected to the scrubber for controlling pressure between theinner cladding casing and the core layer rod; an outer end of the innercladding casing is connected to the core rod hollow shaft hermeticallyat a position outside the vents to communicate the external gas pipewith a gap between the inner cladding casing and the core layer rod; andthe high temperature heat source is arranged outside the inner claddingcasing.
 2. The apparatus of claim 1, wherein an inner diameter of theinner cladding casing is at least 0.3 mm larger than an outer diameterof the core layer rod.
 3. The apparatus of claim 1, wherein the innercladding casing of the inner cladding casing component is disposedoutside the core layer rod of the core rod component, and a distancebetween the inner cladding casing and the core layer rod is 0.15 to 5mm.
 4. The apparatus of claim 1, wherein the gases comprise a purge gas,a dry gas, and an etching gas; the purge gas is purified N₂, O₂, or He;the dry gas is Cl₂; and the etching gas is CF₄, C₂F₆, or SF₆.
 5. Theapparatus of claim 1, wherein the high temperature heat source is a gasblowtorch or furnace.
 6. A method for producing depressed cladding corerod of an ultra-low water peak optical fiber, the method comprising: 1)producing a core rod component: using a glass lathe to fuse and splice acore layer rod and a core rod hollow shaft together; 2) producing aninner cladding casing component: using the glass lathe to fuse andsplice an inner cladding casing and a casing hollow shaft together,wherein an inner diameter of the inner cladding casing is at least 0.3mm larger than an outer diameter of the core layer rod; 3) disposing thecore rod hollow shaft and the casing hollow shaft respectively in twochucks of the glass lathe, wherein a distance between the inner claddingcasing and the core layer rod is 0.15 to 5 mm; 4) cutting offconnections among a pressure controlling pipe, a scrubber, and a vacuumpump, then connecting the core rod hollow shaft to an external gas pipevia a rotary joint, and then connecting the casing hollow shaft to thescrubber; 5) connecting the inner cladding casing to the core rod hollowshaft hermetically; 6) turning on the glass lathe, wherein rotationalspeeds of the two chucks are 20 to 100 rpm; 7) transporting a firstmixture gas comprising a purge gas and a dry gas to the core rod hollowshaft at room temperature for approximate 2 minutes via the external gaspipe, wherein a flow rate ratio of the purge gas to the dry gas is from20:1 to 80:1; 8) while transporting the first mixture gases to the corerod hollow shaft continuously in a flow rate ratio of the purge gas tothe dry gas of from 20:1 to 80:1, moving a high temperature heat sourceback and forth twice to warm the inner cladding casing, wherein a speedof moving the high temperature heat source is 50 to 200 mm per minuteand a temperature of an inner wall of the inner cladding casing isapproximately 300° C. to 800° C.; 9) transporting a second mixture gascomprising the purge gas and an etching gas to the core rod hollow shaftto clean interfaces of glass, wherein a flow rate ratio of the purge gasto the etching gas is 5:1 to 20:1; then moving the high temperature heatsource from one end where the second mixture gas flows in to one endwhere the second mixture gas flows out, wherein the etching gasdecomposes and reacts with the glass at a temperature of 1200° C. to1900° C., and the moving speed of the high temperature heat source is 20to 100 mm per minute; and then adjusting a pressure within the gapbetween the inner cladding casing and the core layer rod to prevent anouter diameter of the inner cladding casing from shrinking or expandingat a high temperature, wherein the pressure within the gap between theinner cladding casing and the core layer rod is from 30 Pa to 400 Pa;10) transporting the first mixture gas to the core rod hollow shaft,wherein the flow rate ratio of the purge gas to the dry gas is 20:1 to80:1, and the pressure within the gap between the inner cladding casingand the core layer rod is lower than 60 Pa; then heating the innercladding casing with the high temperature heat source at the end wheregases flow out, i.e., at the end near the casing hollow shaft, to induceshrinkage of the inner cladding casing; and then opening the pressurecontrolling pipe and fusing the inner cladding casing and the core layerrod together at the end where gases flow out when the gap between theinner cladding casing and the core layer rod disappears; 11)transporting the first mixture gas, wherein the flow rate ratio of thepurge gas to the dry gas is 20:1 to 80:1, and the tube pressure iscontrolled according to the thickness of the inner cladding casing in arange of from +60 Pa to −99 kPa; then opening the connection between thepressure controlling pipe and the vacuum pump when a negative pressureis required, and then moving the high temperature heat source towardsthe end where the gases flow in to heat the inner cladding casing and tofuse the inner cladding casing with the core layer rod, wherein thespeed of moving the high temperature heat source is 5 to 100 mm perminute; and 12) controlling a vacuum degree in the external gas pipe andthe pressure controlling pipe in a range of from +60 Pa to −99 kPa,controlling the temperature of high temperature heat source in a rangeof 1200° C. to 1900° C., and controlling the speed of moving the hightemperature heat source to be 5 to 100 mm per minute to fuse the innercladding casing with the core layer rod to provide a depressed-claddingcore rod not having interfacial air bubbles and air lines.
 7. The methodof claim 6, wherein the purge gas is purified N₂, O₂, or He; the dry gasis Cl₂; the etching gas is CF₄, C₂F₆, or SF₆.